U.S. Pat. No. 4,015,618, owned by the same assignee as the present application, discloses prior art apparatus for automatically mixing and circulating a washing solution through dairy equipment. That apparatus comprises detergent supply pumps for adding detergents to water to form the washing solution which is then circulated by a circulating pump through the equipment. That apparatus comprises a control system, including a timer, to periodically operate the detergent supply pumps and the circulating pump in a predetermined sequence. That control system further includes an electrical probe for sensing the pH level of the washing solution to ascertain the amount of detergent being added and for controlling operation of the detergent pumps accordingly. The prior art apparatus presumes that the detergent supply will be properly maintained so there is no failure of supply during a wash cycle. However, it is not always possible to ensure adequate supply or to anticipate system malfunctions that might affect availability of the detergent supply. Thus, if the detergent supply fails for any reason and goes undetected, that particular wash cycle is carried out with insufficient detergent in the washing solution, resulting in improper cleaning of the equipment. Furthermore, there is a risk that successive wash cycles using an inadequate washing solution will be carried out undetected.